Traditionally, photographs have been created by capturing a latent image on silver halide sensitised film, and then transforming the latent image into a colour reproduction of the original subject by a series of chemical processes. Overall the process is characterised by a continuous (analogue) relationship between an original image and the resulting print. Prints produced in this way have acceptable stability to various invasive agents such as fingergrease and also to fading by light.
In recent years the advent of digital cameras has allowed the capture of images in an electronic digital form, and their subsequent manipulation and printing without recourse to the use of silver films and associated chemical processing. This offers the possibility of substantial control over the whole process, which can now be performed in the home or office by a wide portion of the general population. It is predicted by some that digital imaging will in time substantially replace traditional photographic methods.
Printing processes used for digital imaging include thermal dye transfer printing and ink jet printing.
Thermal dye transfer printing is a generic term for processes in which one or more thermally transferable dyes arc caused to transfer from a dyesheet to a receiver in response to thermal stimuli. Using a dyesheet comprising a thin substrate supporting a dyecoat containing one or more such dyes uniformly spread over an entire printing area of the dyesheet, printing can be effected by heating selected discrete areas of the dyesheet while the dyecoat is pressed against a receiver sheet, thereby causing dye to transfer to corresponding areas of that receiver. The shape of the pattern transferred is determined by the number and location of the discrete areas which are subjected to heating. Full colour prints can be produced by printing with different coloured dyecoats sequentially in like manner, and the different coloured dyecoats are usually provided as discrete uniform print-size areas in a repeated sequence along the same ribbon-like dyesheet.
High resolution photograph-like prints can be produced by thermal dye transfer printing using appropriate printing equipment, such as a programmable thermal print head or laser printer, controlled by electronic signals derived from video, computer, electronic still camera, or similar signal generating apparatus. A typical high speed thermal print head has a row of individually operable tiny heaters spaced to print six or more pixels per millimetre, using very short hot pulses.
Receiver medium for thermal dye transfer printing generally comprises a substrate sheet supporting a receiver coat of a dye-receptive composition containing a material having an affinity for the dye molecules, and into which they can readily diffuse when an area of dyesheet pressed against it is heated during printing. Such receiver coats are typically around 2 to 6 μm thick, and materials with good dye-affinity are generally thermoplastic polymers, such as saturated polyesters, soluble in common solvents to enable them readily to be coated onto the substrate from solution.
In ink jet printing, a steam of charged ink droplets is projected onto ink receptive receiver medium at high velocity, eg up to 20 m/s. Movement of the ink jet may be computer controlled, and images may be formed and printed rapidly. By using inks of different colours a full colour image can be produced. In general, ink jet printing inks are water-based compositions that are usually dye-based solutions. Such inks are widely used in a range of ink jet printers, for commercial, office and domestic use including desk-top printers. A receiver medium for use in ink jet printing generally comprises a substrate carrying an ink absorbent layer that typically comprises a polymer or a mixture of polymers, eg cellulosic polymers such as carboxymethyl cellulose and especially hydroxyethyl cellulose; gelatins; vinyl polymers such as a polyvinyl alcohol and polyvinyl pyrrolidone; and acrylic polymers such as polyacrylic acid.
For convenience, dyes and inks for use in digital imaging techniques will be referred to generally as dyes.
Both the above printing methods suffer from the fact that the resulting images generally comprise dyes kinetically frozen in polymeric layers which are at a temperature below their Tg, ie the dye molecules are physically entrapped in the polymer in the form of a solid solution and are not chemically bound in position. Any changes resulting in the system being above the polymer Tg, eg due to thermal energy or ingress of swellants, plasticisers or contaminants such as fingergrease, are liable to result in dye migration which in turn can lead to dye crystallisation and loss of colour density or image blurring and loss of resolution.
It is also known in the art of digital imaging, and in particular thermal dye transfer printing and ink-jet imaging technologies, that dyes printed into low Tg polymeric receiver materials suffer image degradation as a result of dye migration over time. To overcome this, most current imaging systems employ receiver media with relatively high Tg polymers which are stable at room temperature, typically having a Tg of at least 60° C. High Tg receiver systems, however, typically suffer from low dye diffusion rates during the printing process, which in turn results in high dye concentrations near the surface of the image and subsequently poor image stability with respect to light and physical contact.
One approach to improving image stability in thermal dye transfer printing is to apply a polymeric protective layer either as a separate panel from a thermal transfer dyesheet, or in a separate lamination step. However, this approach can result in image blurring caused by thermal stimulation on application of the layer and also has the drawback of requiring extra material in the form of a separate panel or lamination sheet.
Another approach to improving image stability in thermal dye transfer printing is to arrange for the image dyes to be interactive with the receiver layer. For example, it has been proposed to render dyes immobile by the following methods:
1) Acid-base interactions. Here the dyes contain eg two or more acid groups that can form strong interactions with basic groups attached to the receiver polymer, eg as described in WO 96/34766.
2) Chelation. U.S. Pat. No. 5,512,531 of Konica discloses the interaction of dyes with metal ions in the receiver by a chelation process.
3) Cationic dyes. EP 506,034 of Sony describes the use of cationic dyes with receivers containing layered inorganic materials. These layered materials are capable of absorbing the charged cationic dyes, and holding them in a stable environment.
Finally, performance may be improved by reacting an appropriate dyestuff with the receiver layer, resulting in the formation of a covalent bond to attach the dye to the receiver.
It is a feature of all these interactive methods that insufficient heat is available to allow the dyes to migrate sufficiently to permit the development of the full interaction, and hence to realise the full potential stability of the system. Indeed stratification of the dye at the surface (ie concentration of dye at the surface of the receiver medium) is a particular problem with these systems. The significance of this problem has led to proposals to use acid vapour treatment to fix an image (U.S. Pat. No. 4,880,769), and to use low Tg receiver polymers containing organic acid groups or low Tg receivers containing oligomeric and polymeric acids to reprotonate deprotonated cationic dyes (U.S. Pat. No. 5,627,956).